[C#] Felder ausfüllen auf einer Webseite

<input type="hidden" name="mainPagePart" value="mainPagePart" />

            <!-- START PIN -->
            <fieldset><div id="mainPagePart:j_id51" class="form-inner-pin"><div id="mainPagePart:j_id52" class="col-left1"><div id="mainPagePart:j_id53" class="row pin">
                            <label>PIN-Code:</label></div><div id="mainPagePart:j_id57" class="row pin"><div id="mainPagePart:j_id58" class="form-block"><span class="text text-small3"><input id="mainPagePart:rn1" type="text" name="mainPagePart:rn1" autocomplete="off" class="text" maxlength="4" onkeyup="return autoTab(this, 4, event);" size="4" style="text-align:center" /></span><span class="text text-small3"><input id="mainPagePart:rn2" type="text" name="mainPagePart:rn2" autocomplete="off" class="text" maxlength="4" onkeydown="return autoBack(this, event);" onkeyup="return autoTab(this, 4, event);" size="4" style="text-align:center" /></span><span class="text text-small3"><input id="mainPagePart:rn3" type="text" name="mainPagePart:rn3" autocomplete="off" class="text" maxlength="4" onkeydown="return autoBack(this, event);" onkeyup="return autoTab(this, 4, event);" size="4" style="text-align:center" /></span><span class="text text-small3"><input id="mainPagePart:rn4" type="text" name="mainPagePart:rn4" autocomplete="off" class="text" maxlength="4" onkeydown="return autoBack(this, event);" size="4" style="text-align:center" />

        private void check_Click(object sender, EventArgs e)
        {
            for (int i = 0; i < webBrowser1.Document.Forms.Count; i++)
            {
                HtmlElement element = webBrowser1.Document.Forms[i].All["mainPagePart"];
                Thread.Sleep(200);
                if (element != null && element.GetAttribute("").Equals(""))
                {
                    webBrowser1.Document.All["mainPagePart:rn1"].SetAttribute("text", id1.Text);//Pin Teil 1
                    webBrowser1.Document.All["mainPagePart:rn2"].SetAttribute("text", id2.Text);
                    //webBrowser1.Document.Forms[i].InvokeMember("submit"); //Drückt Absenden!
                }

            }